The application of synthetic polymer chemistry to the field of sports equipment has revolutionized the performance of athletes in many sports. One sport in which this is particularly true is golf, especially as relates to advances in golf ball performance and ease of manufacture. For instance, the earliest golf balls consisted of a leather cover filled with wet feathers. These “feathery” golf balls were subsequently replaced with a single piece golf ball made from “gutta percha,” a naturally occurring rubber-like material. In the early 1900's, the wound rubber ball was introduced, consisting of a solid rubber core around which rubber thread was tightly wound with a gutta percha cover.
More modern golf balls can be classified as one-piece, two-piece, and three-piece. One-piece balls are molded from a homogeneous mass of material upon which is molded a dimple pattern. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid rubber core. These are the most popular types of balls in use today. In attempts to further modify the ball performance, especially in terms of the distance such balls travel, and the feel transmitted to the golfer through the club on striking the ball, the basic two-piece ball construction has been further modified by the introduction of additional layers between the core and outer cover layer. If one additional layer is introduced between the core and outer cover layer, a so called “three-piece ball” results, and similarly, if two additional layers are introduced between the core and outer cover layer, a so called “four-piece ball” results, and so on.
Conventionally, golf ball cover and intermediate layers are positioned over a core or other internal layer using one of three methods: casting, injection molding, or compression molding. Injection molding generally involves using a mold having one or more sets of two hemispherical mold sections that mate to form a spherical cavity during the molding process. The pairs of mold sections are configured to define a spherical cavity in their interior when mated. When used to mold an outer cover layer for a golf ball, the mold sections can be configured so that the inner surfaces that mate to form the spherical cavity include protrusions configured to form dimples on the outer surface of the molded cover layer. The mold sections are connected to openings, or gates, evenly distributed near or around the parting line, or point of intersection, of the mold sections through which the material to be molded flows into the cavity. The gates are connected to a runner and a sprue that serve to channel the molding material through the gates. When used to mold a layer onto an existing structure, such as a ball core, the mold includes a number of support pins disposed throughout the mold sections. The support pins are configured to be retractable, moving into and out of the cavity perpendicular to the spherical cavity surface. The support pins maintain the position of the core while the molten material flows through the gates into the cavity between the core and the mold sections. The mold itself may be a cold mold or a heated mold. In the case of a heated mold, thermal energy is applied to the material in the mold so that a chemical reaction may take place in the material. Thermoset materials have desirable mechanical properties, and hence would be beneficial to producers of golf balls using this process. Unfortunately, thermoset materials generally are not well suited for injection molding. As the reactants for thermoset polyurethane are mixed, they begin to cure and become highly viscous while traveling through the sprue and into the runners of the injection mold, leading to injection difficulties. For this reason, thermoset materials typically are formed into a ball layer using a casting process free of any injection molding steps.
In contrast to injection molding, which generally is used to prepare layers from thermoplastic materials, casting often is used to prepare layers from thermoset material (i.e., materials that cure irreversibly). In a casting process, the thermoset material is added directly to the mold sections immediately after it is created. Then, the material is allowed to partially cure to a gelatinous state, so that it will support the weight of a core. Once cured to this state, the core is positioned in one of the mold sections, and the two mold sections are then mated. The material then cures to completion, forming a layer around the core. The timing of the positioning of the core is crucial for forming a layer having uniform thickness. The equipment used for this positioning is costly, because the core must be centered in the material in its gelatinous state, and at least one of the mold sections, after having material positioned therein, must be turned over and positioned onto its corresponding mold section.
Compression molding a ball layer typically requires an initial step of making half shells by injection molding the layer material into a cold injection mold. The half shells then are positioned in a compression mold around a ball core, whereupon heat and pressure are used to mold the half shells into a complete layer over the core. Compression molding also can be used as a curing step after injection molding. In such a process, an outer layer of thermally curable material is injection molded around a core in a cold mold. After the material solidifies, the ball is removed and placed into a mold, in which heat and pressure are applied to the ball to induce curing in the outer layer.
Reaction injection molding is a processing technique used specifically for certain reactive thermosetting plastics. As mentioned above, by “reactive” it is meant that the polymer is formed from two or more components which react. Generally, the components, prior to reacting, exhibit relatively low viscosities. The low viscosities of the components allow using lower temperatures and pressures than those utilized in traditional injection molding. In reaction injection molding, the two or more components are combined and reacted to produce the final polymerized material. Mixing these separate components is critical, a distinct difference from traditional injection molding. The process of reaction injection molding a golf ball cover involves placing a golf ball core into a die, closing the die, injecting the reactive components into a mixing chamber where they combine, and transferring the combined material into the die. The mixing begins the polymerization reaction which is typically completed upon cooling of the cover material.
Finally, the mold material itself and any supporting pins and vent pins may be made at least in part from a porous metal material. The porous metal is suitable for use as part of an injection or compression mold. The porous metal can be used for all or part of the mold sections defining the spherical cavity. Other regions of the mold that can advantageously be made from porous metal include pins, runners, sprues, and any other parts of the mold which come into contact with the material from which the ball portions are formed. Full details of this mold design are disclosed in U.S. Pat. No. 6,776,942 to H. J. Kim, the entire contents of which are herein incorporated by reference.
Balata had been used as the primary material for covers of golf balls until the 1960's when SURLYN®, an ionomeric resin made by E.I. DuPont de Nemours & Co., was introduced to the golf industry. SURLYN® costs less than balata and has a better cut resistance than balata. At the present time, SURLYN® is used as the primary source of cover stock for most two-piece and some three-piece golf balls. The problem with SURLYN®-covered golf balls, however, is that they lack the “click” and “feel” which golfers had become accustomed to with balata. “Click” is the sound made when the ball is hit by a golf club while “feel” is the overall sensation imparted to the golfer when the ball is hit. However, unlike SURLYN®-covered golf balls, polyurethane- or polyurea-covered golf balls can be made to have the “click” and “feel” of balata.
Polyurethanes or polyureas typically are prepared by reacting a diisocyanate with a polyol (in the case of polyurethanes) or with a polyamine (in the case of a polyurea). Thermoplastic polyurethanes or polyureas may consist solely of this initial mixture or may be further combined with a chain extender to vary properties such as hardness of the thermoplastic. Thermoset polyurethanes or polyureas typically are formed by the reaction of a diisocyanate and a polyol or polyamine respectively, and an additional crosslinking agent to crosslink or cure the material to result in a thermoset.
In what is known as a one-shot process, the three reactants, diisocyanate, polyol or polyamine, and optionally a chain extender or a curing agent, are combined in one step. Alternatively, a two-step process may occur in which the first step involves reacting the diisocyanate and the polyol (in the case of polyurethane) or the polyamine (in the case of a polyurea) to form a so-called prepolymer, to which can then be added either the chain extender or the curing agent. This procedure is known as the prepolymer process.
In addition, although depicted as discrete component packages as above, it is also possible to control the degree of crosslinking, and hence the degree of thermoplastic or thermoset properties in a final composition, by varying the stoichiometry not only of the diisocyanate-to-chain extender or curing agent ratio, but also the initial diisocyanate-to-polyol or polyamine ratio. Of course in the prepolymer process, the initial diisocyanate-to-polyol or polyamine ratio is fixed on selection of the required prepolymer.
Of the two processes, the prepolymer process is thus preferred since it allows for greater control over the reaction. Nevertheless, golf balls in accordance with the present invention can be produced using either process.
In view of the aforementioned advantages of polyurethane and polyurea as a golf ball cover component, numerous patents have disclosed various formulations for these materials. For example, Hewitt, et al., U.S. Pat. No. 4,248,432 discloses a thermoplastic polyester urethane golf ball cover formed from a reaction product of a polyester glycol (formed from an aliphatic diol and an aliphatic dicarboxylic acid) with para-phenylene diisocyanate (PPDI) or cyclohexane diisocyanate, in the substantial absence of curing or crosslinking agents.
U.S. Pat. No. 4,123,061 teaches that a golf ball can be made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol or a diamine. The specific diamines taught by the '061 patent are 3,3′-dichlorobenzidene; 3,3′-dichloro-4,4′-diaminodiphenyl methane (MOCA); N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine and Curalon L, a trade name for a mixture of aromatic diamines sold by Uniroyal, Inc.
U.S. Pat. No. 3,989,568 teaches a three-component system employing either one or two polyurethane prepolymers and one or two curing agents. Both polyol and diamine curing agents are taught by the '568 patent.
Dusbiber, U.S. Pat. No. 4,123,061 discloses a polyurethane golf ball cover prepared from the reaction product of a polyether, a diisocyanate and a curing agent. The polyether may be polyalkylene ether glycol or polytetramethylene ether glycol, and that the diisocyanate may be TDI, 4,4″-diphenylmethane diisocyanate (MDI), and 3,3″-dimethyl-4,4″-biphenylene diisocyanate (TODI). Additionally, the curing agent may be either a polyol (either tri- or tetra-functional and not di-functional) such as triisopropanol amine (TIPA) or trimethylol propane (TMP), or an amine-type having at least two reactive amine groups.
Holloway, U.S. Pat. No. 4,349,657 discloses a method for preparing polyester urethanes with PPDI by reacting a polyester (e.g. prepared from aliphatic glycols having 2-8 carbons reacted with aliphatic dicarboxylic acids having 4-10 carbons) with a molar excess of PPDI to obtain an isocyanate-terminated polyester urethane (in liquid form and stable at reaction temperatures), and then reacting the polyester urethane with additional polyester.
Wu, U.S. Pat. No. 5,334,673 discloses a polyurethane prepolymer cured with a slow-reacting curing agent selected from slow-reacting polyamine curing agents and difunctional glycols (i.e., 3,5-dimethylthio-2,4-toluenediamine, 3,5-dimethylthio-2,6-toluenediamine, N,N″-dialkyldiamino diphenyl methane, trimethyleneglycol-di-p-aminobenzoate, polytetramethyleneoxide-di-p-aminobenzoate, 1,4-butanediol, 2,3-butanediol, 2,3-dimethyl-2,3-butanediol, ethylene glycol, and mixtures of the same).
Wu, U.S. Pat. No. 5,484,870 discloses golf balls having covers composed of a polyurea composition. The polyurea composition disclosed is a reaction product of an organic isocyanate having at least two functional groups and an organic amine having at least two functional groups. One of the organic isocyanates disclosed is PPDI.
Finally, in addition to discrete thermoplastic or thermoset materials, it also is possible to modify a thermoplastic polyurethane or polyurea composition by introducing materials in the composition that undergo subsequent curing after molding the thermoplastic to provide properties similar to those of a thermoset. For example, Kim in U.S. Pat. No. 6,924,337, the entire contents of which are hereby incorporated by reference, discloses a thermoplastic urethane or urea composition optionally comprising chain extenders and further comprising a peroxide or peroxide mixture, which can then undergo post curing to result in a thermoset.
Also, Kim et al. in U.S. Pat. No. 6,939,924, the entire contents of which are hereby incorporated by reference, discloses a thermoplastic urethane or urea composition, optionally also comprising chain extenders, that is prepared from a diisocyanate and a modified or blocked diisocyanate which unblocks and induces further cross linking post extrusion. The modified isocyanate preferably is selected from the group consisting of: isophorone diisocyanate (IPDI)-based uretdione-type crosslinker; a combination of a uretdione adduct of IPDI and a partially e-caprolactam-modified IPDI; a combination of isocyanate adducts modified by e-caprolactam and a carboxylic acid functional group; a caprolactam-modified Desmodur diisocyanate; a Desmodur diisocyanate having a 3,5-dimethylpyrazole modified isocyanate; or mixtures of these.
Finally, Kim et al. in U.S. Pat. No. 7,037,985 B2, the entire contents of which are hereby incorporated by reference, discloses thermoplastic urethane or urea compositions further comprising a reaction product of a nitroso compound and a diisocyanate or a polyisocyanate. The nitroso reaction product has a characteristic temperature at which it decomposes to regenerate the nitroso compound and diisocyanate or polyisocyanate. Thus, by judicious choice of the post-processing temperature, further crosslinking can be induced in the originally thermoplastic composition to provide thermoset-like properties.
Although the prior art has disclosed golf ball covers composed of many different polyurethane materials, none of these golf balls have proven completely satisfactory. A particular dissatisfaction has been the yellowing of thermoset or thermoplastic polyurethane or polyurea covers upon exposure to sunlight (ultraviolet radiation). Because the polyureas or polyurethanes used to make the covers of such golf balls generally contain an aromatic component, e.g., aromatic diisocyanate, polyol, or polyamine, they are susceptible to discoloration upon exposure to light, particularly ultraviolet (UV) light. To slow down the discoloration, light and UV stabilizers, e.g., TINUVIN® 770, 765, and 328, are added to these aromatic polymeric materials. However, to further ensure that the covers formed from aromatic polyurethanes do not appear discolored, the covers are painted with white paint and then covered with a clear coat to maintain the white color of the golf ball. The application of a uniform white pigmented coat to the dimpled surface of the golf ball is a difficult process that adds time and expense to the golf ball manufacturing process. In addition, when the paint and clear coat layers are compromised during play, due to scuffing or shear generated by the clubface, the UV protection afforded by the paint layer is compromised, which can lead to ball yellowing on further play.
Dewanjee, U.S. Pat. Nos. 6,592,472 and 6,974,854 discloses golf balls with covers composed of thermosetting polyurethane purportedly having increased resistance to yellowing. This was accomplished by providing a cover composed of a thermosetting polyurethane material formed from a toluene diisocyanate prepolymer and a curative composed of 20 to 40 parts 4,4″-methylenebis-(3-chloro,2,6-diethyl)-aniline and 80 to 60 parts diethyl 2,4-toluenediamine.
Also, U.S. Pat. No. 7,041,769, entitled “Polyurethane Compositions for Golf Balls,” states that “[f]urthermore, because the polyurethanes and polyurea used to make the covers of such golf balls generally contain an aromatic component, e.g., aromatic diisocyanate, polyol or polyamine, they are susceptible to discoloration upon exposure to light, particularly ultraviolet (UV) light.” U.S. Pat. No. 7,041,769, column 2, lines 21-26. The '769 patent discloses forming polycarbonate polyols, e.g., HO—[R—OCOO—]nR—OH.
However, there remains a need for thermoset or thermoplastic polyurethane or polyurea covers that have reduced yellowing relative to known compositions. These compositions also have the other desired physical properties useful for making golf balls that are provided by conventional polyurea or polyurethane compositions.